Electro-magnetic radiations (EMR) emitted by higher harmonics of a basic clock frequency of a digital circuit due to the activities at the clock edges significantly effect the normal operation of neighboring, circuits. To prevent the adverse effects of such EMR the amplitude of the emitted radiations should be kept to a minimum. Various methods have been adopted for reducing the amplitude of interfering radiations. For example, the phase/frequency of the basic clock frequency is modulated for spreading the emitted radiation frequency band for distributing the radiating energy into adjacent side-bands about each harmonic frequency. Consequently, this reduces the amplitude of higher harmonic frequencies.
Frequency modulation is a predominantly analog technique and has overheads in terms of implementations in the integrated circuit (IC). Moreover, any arbitrary modulation does not generate an optimum EMI reduction. In other words, randomly modulating a clock generating source may not provide improved attenuation of EMI. A proper selection of the modulation is essential to ensure its effectiveness in reducing EMR. Reference is directed to Spread Spectrum Clock Generation for the Reduction of Radiated Emissions, Hardin et al., IEEE International Symposium on Electromagnetic Compatibility, 22-26 Aug., 1994. U.S. Pat. Nos. 6,366,174; 6,373,306; and 6,351,485 propose the use of PLLs for reducing EMR. However, these techniques cannot be used for systems in which the PLL is a procured object, that is, one which cannot be modified.
Prior art digital modulation of the clock period, as disclosed in U.S. Pat. No. 5,442,664 for example, proposes the use of ‘n’ number of delay components of equal delay Δ, as shown in FIG. 1. Selection control 3 (up/down counter of n-states) uses delay tap selection logic 2 to modulate the time period of the clock. It makes the clock period (Tck+Δ) for the first ‘n’ clock cycles and (Tck−Δ) for the next ‘n’ clock cycles alternating in this manner continuously. This technique does not provide optimum spread spectrum EMR reduction since the energy concentrates around two frequencies: 1/(Tck+Δ) and 1/(Tck−Δ) only.
U.S. Patent Publication No. 20010045857 proposes a similar scheme, differing only in that it provides a technique to adjust (Δ/Tck) by adjusting the supply voltage to the delay units. This technique is not suitable for most integrated digital systems since it calls for varying the supply voltage.
U.S. Pat. No. 5,699,005 proposes a technique utilizing both equal delay and unequal delays in the delay chain block, but effectively the successive increment (decrement) of the delay in the delay chain are always constant (Δ). The main difference lies in the way the switching of the delay taps onto the clock output is performed. Here, the delay tap selection is done using a pseudo-random number sequence, which causes radiation energy spectrum to be better distributed as compared to the previous techniques. However, the pseudo-random nature results in a sharp odd time period variation while other time periods are very slow. This results in the concentration of the radiated frequency around certain frequencies instead of spreading as evenly as possible.